Here is a screenshot of Process Explorer, showing Firefox using a staggering 80% of cpu whilst running a HTML5 1080p video.

Are you using NVIDIA? It's because they disabled hardware acceleration thus using more processing power.

[[Hardware acceleration in Windows 10 is not working - NVIDIA|https://support.mozilla.org/en-US/questions/1077259]]

That must be the explanation. I have the following NVidia card:

NVidia GeForce GT 960 2Gb with the 353.62 drivers.

For a comparison, here is the cpu use when viewing the same HTML5 1080p video in Edge.

I take it enabling hardware acceleration in the FFox settings has no effect on the high FFox cpu usage. This now means that Opera is going to have to be used as my default browser until this is fixed.

Polochamps said

Are you using NVIDIA? It's because they disabled hardware acceleration thus using more processing power. [[Hardware acceleration in Windows 10 is not working - NVIDIA|https://support.mozilla.org/en-US/questions/1077259]]

If this is the case, why is FF misleading many Windows 10 Firefox users with an upgrade to FF 40.0, selling it as ensuring a smooth transition for anyone that has upgraded their system to Windows 10. It's a blatant, misleading untruth and FF should remove the text from their F 40.0 upgrade spiel.

Why not upgrade to FF 40, it'll ensure that many Windows 10 users will experience extremely high CPU ratings and a complete operating system slowdown. That is the fact of the matter. Mozilla is misleading Windows 10 users with NVidia cards with this upgrade to FF 40. It is NOT Windows 10 compatible. FF should stop trying to sell it as though it is.

Well, the new Windows 10 355.60 drivers from Nvidia, which were released today, have done nothing to fix HTML5 playback and the totally out-of-order high CPU in FF. 83% cpu from FF, these 355.60 drivers are worse than the last.

I can't stick with FF with this amateur nonsense with Nvidia drivers. What is the problem now with hardware acceleration? Astounding that it's not been fixed with these new and so-called Windows 10 drivers for desktops.

Double post, see above post.

I can confirm that it still didn't fix the issue with the new drivers but unlike before, I can force it now.

http://i.imgur.com/5jAPjhj.jpg

If you want to try this, please do it at your own risk.

about:config

- webgl.force-enabled=true. - webgl.msaa-force=true. - layers.acceleration.force-enabled=true. - gfx.direct2d.force-enabled=true.

Polochamps said

I can confirm that it still didn't fix the issue with the new drivers but unlike before, I can force it now. http://i.imgur.com/5jAPjhj.jpg If you want to try this, please do it at your own risk. about:config - webgl.force-enabled=true. - webgl.msaa-force=true. - layers.acceleration.force-enabled=true. - gfx.direct2d.force-enabled=true.

Thanks for the tip. It's a pity Nvidia are so hopeless at times. Nonetheless, I'm sure they are working on a solution.

I'll alter my config settings when I got home from work later today. I'll see if it works. If not, I'll change them back to my current default ones and just keep using Flash as the default player on youtube as a temporary fix.

Good news, this now seems to be fixed in FF 40.0.2 when using the latest Nvidia Windows 10 355.60 drivers . CPU levels are back to normal when playing 1080p, and other content, when using the HTML5 player.

I think we can now call this solved.

Well it's not fixed here - latest Nvidia drivers (as at 10/09/2015) for GTX970, firefox 40.0.3 and just now when doing general browsing, firefox was using 30% of my CPU.. wouldn't think that would be a problem, but I was getting total browser lockups and firefox ended up crashing not once, but twice just now. I am going to look for another browser, it is no good if I lose browsing sessions.. running on a Haswell i7 3.4GHz with 8GB DDR3 memory. Also worth mentioning that firefox memory leaks are pretty bad too.. If I browse the internet for an hour or 2, the memory usage can go up to around 1.8GB, and even closing down all tabs except 1 does not reduce memory usage.

What's the news on this, is there a fix? Who is at fault Win10, FF, nvidia? Both Integrated Graphics intel HD4000 / nvidia GeForce GTX 680M v355.82 & 358.50-notebook Youtube HTML5 video locks up FireFox to the point of crashing. Even a Youtube TAB that is open (no video playing) will cause FF to cry.

Win 7 + FF does not have this problem.